Summary: The goal of this study is to develop and validate a new in vitro potency test for rabies virus vaccine release. To initiate this project, a workshop was held at CBER, September 25 and 26, 2000. Participants included scientists from the FDA, CDC, rabies vaccine manufacturers, WHO, and European regulatory agencies. The outcome of this meeting was that four laboratories were chosen to work on the development of an in vitro potency test for rabies vaccines. We have worked to develop a capture ELISA which uses polyclonal sheep antiserum generated against purified rabies G protein as the capture antibody. Vaccine preparations, including a reference vaccine are diluted and bound to the capture antibody. A rabies G protein specific monoclonal antibody which has been shown to be protective is used as a secondary antibody to measure the amount of antigen bound. Potency is defined by comparing the amount of test vaccine antigen bound to the reference vaccine bound. A second working group meeting was held in January 2001 at Aventis Pasteur (Lyon, France). After comparing capture ELISA protocols and results from each of the collaborative laboratories, a single protocol was chosen and a specific set of assay reagents were identified. It was determined that the second phase of test development would be to have the four collaborative labs use a single protocol and uniform assay reagents to test multiple product lots from each manufacturer. These reagents have been shared between laboratories and we have begun testing the lots. We have received many lots of vaccine product and are testing these with the new ELISA based potency test and with the traditional animal based NIH potency test. Using the reagents defined in the unified protocol, we have shown that the ELISA test has very low intratest and intertest variability in our laboratory. We are completing the animal based testing to determine if a correlation exists between the two types of tests. In addition, we have received vaccine lots with a subpotent titer. We will test these to see if the ELISA test will measure low potency as measured by the animal potency test. We will meet with collaborators in the Fall of 2002 to compare results and study the variability of testing identical product lots between the collaborative test laboratories. If results are consistent between the laboratories involved in this set of tests, we will move forward to set up a large scale collaborative study to determine the suitability of the ELISA test as a replacement test for the NIH animal potency test.